The casting of metal articles using sand molds, sand shells and sand cores is well known in the art. Detailed information regarding the state of this technology can be found, for example, in a text by James P. LaRue, EdD, Basic Metalcasting (The American Foundrymen's Society, Inc., Des Plaines, Ill., 1989). Using such a technique, a mold can be made from a mixture of sand and (typically) an organic binder by packing the mixture loosely or tightly around a pattern. The pattern is then removed, leaving a cavity in the sand which replicates the shape of the pattern. Once the organic binder is shape-stabilized by any of a number of hardening techniques (as described below), the cavities in the sand mold are filled with molten metal by pouring the molten metal into the mold.
In a typical shell molding operation, binder-coated sand can be blown onto the interior surface of a heated metal pattern. In a relatively short time (20-30 seconds) the heat from the pattern penetrates the sand, producing a bond in the heat-affected layer. This layer clings to the pattern, and when the pattern is rotated, the sand not affected by the heat falls into a hopper for further use. The thin, bonded layer of binder-coated sand clinging to the pattern is then cured by heating. The cured shell is then pushed from the pattern by ejector pins. When a mating shell is produced, the shells are aligned and fastened together with a high-temperature adhesive for pouring.
Just as the sand mold cavity provides the external shape of a casting, any holes or other internal shapes in a casting can be produced by using sand cores. When such cores are made from sand, numerous acceptable processes for making these cores are acceptable. In most cases, a sand mixture comprising a binder material is placed into a corebox. There, the sand mixture takes the shape of the cavity in the box, becomes hard, and is removed. After the mold is made, the core is then set in the "drag" just before the mold is closed. When the metal is poured, the molten metal fills the mold cavity except for where sand cores are present. Thus, the shape of the solidified casting results from the combined shapes of the mold and the sand core(s).
Before 1943, coremaking was simple. There was one core process, known as oil-sand, which had been used for many years. Since then, there has been a dramatic increase in coremaking technology. At present there are at least 21 different coremaking systems. Over 160 binder materials are now available for making cores. These binder materials can be categorized as vapor-cured (cured by a gas of some kind), heat-cured (cured by heat), or no-bake (cured by chemical reaction).
While it is not the intent of this disclosure to discuss all of the various binders which are currently in use for such processes, perhaps the most commonly utilized binders comprise both inorganic and organic resins.
In the realm of inorganic systems, both vapor-cured and no-bake sodium silicate binders are known. No-bake, oxide-cured phosphate binders are also available. Such inorganic binders often have low emissions resulting from their high char forming characteristics. The term "char" should be understood as meaning the solid products of binder decomposition which remain after thermal treatment during the metalcasting process. They do, however, have certain disadvantages.
Vapor-cured sodium silicate binders, for example, are typically processed by coating sand grains with the sodium silicate binder, backing the mixture into a corebox, and then gassing the mixture in the corebox with carbon dioxide for a short period of time (about 10 seconds). This treatment hardens the core, allowing it to be removed from the corebox. One advantage of this system is that the core can be used immediately. A major disadvantage of such systems, however, is the tendency for the resulting cores to absorb moisture. Many of the inorganic resin systems currently in use share this problem.
By far, the largest number of sand binders which are used in the art of metalcasting are organic resins. Vapor-cured systems include the phenolic urethane/amine binders, phenolic esters, furan/peroxide systems which, typically, are acid cured, and epoxy/sulfur dioxide systems. Heat-cured systems include phenolic resins, furan systems, and urea formaldehyde binders. No-bake systems comprise acid-cured furan systems, acid-cured phenolic resins, alkyd oil urethanes, phenolic urethanes, and phenolic esters. While these wholly organic systems often offer flexibility in processing (e.g., these systems can be solvent processed, melted, etc.), the hardened molds or cores produced using such binders have very serious drawbacks including, for example, the evolution of toxic emissions during the metal casting process due to the low char yield characteristics of organic resins.
Additionally, when such binders are used to bond particles together to make shapes directly, similar problems to those discussed above also result. For example, similar problems can occur when making brake shoes, brake pads, clutch parts, gravity wheels, polymer concrete, refractory patches, liners, preforms of various components for further processing, etc.
Organometallic, ceramic precursors are known in the art of ceramic processing. These materials can be in the form of either solvent-soluble solids, meltable solids, or hardenable liquids, all of which permit the processibility of their organic counterparts in the fabrication of ceramic "green bodies". During the sintering of such green parts, however, the ceramic precursor binders have the added advantage of contributing to the overall ceramic content of the finished part, because the thermal decomposition of such ceramic precursor binders results in relatively high yields of ceramic "char". Thus, most of the precursor is retained in the finished part as ceramic material, and very little mass is evolved as undesirable volatiles. This second feature is advantageous, for example, in reducing part shrinkage and the amount of voids present in the fired part, thereby reducing the number of critically sized flaws which have been shown to result in strength degradation of formed bodies.
Such precursors can be monomeric, oligomeric, or polymeric and can be characterized generally by their processing flexibility and high char yields of ceramic material upon thermal decomposition (i.e. pyrolysis). These precursors are neither wholly inorganic nor wholly organic materials, since they comprise metal-carbon bonds. These precursors can be distinguished from other known inorganic binders for sand mold fabrication described above (which comprise no carbon), and other known organic binders (which comprise no metallic elements). It has been unexpectedly discovered that such organometallic "hybrids" which are hardenable liquids are uniquely suited for use as binders for sand grains in the fabrication of sand molds, cores, and shells, since they can provide excellent mold strength at extremely low binder levels. Their utility resides in a unique combination of, for example, the processing flexibility afforded by organic binders and the high char forming characteristics and improved adhesion to sand of inorganic binders. Such binders can therefore be easily processed to provide a hardened sand mold, and subsequently used for metalcasting with a minimum of toxic volatiles being evolved. Further, it has been unexpectedly discovered that such organometallic "hybrids" are uniquely suited for use as binders for filler materials in the fabrication of preforms to be used in the formation of composite materials. For example, such organometallic "hybrids" have been found to be uniquely suited to the formation of metal matrix composites by molten metal infiltration processes (e.g., spontaneous infiltration, pressure and vacuum assisted infiltration, etc.). Moreover, these organometallic "hybrids" have also been found to be useful as preform binders for ceramic matrix composite formation processes (e.g., directed metal oxidation, sintering, isostatic pressing, chemical vapor infiltration, etc.). Further, since such organometallic, ceramic precursor binders are also liquids, they can be employed directly without use of a solvent. This obviates the emissions and disposal problems associated with solvent-based systems which require a "drying" step subsequent to mold shaping.
Siloxanes have been used in the past to improve the adhesion of such binder systems as polycyanoacrylates to sand grains (see, for example, U.S. Pat. No. 4,076,685). In such a system the siloxane is used as a processing aid rather than the binder itself. Additionally, partial condensates of trisilanols have been used in combination with silica as binder systems which are provided in aliphatic alcohol-water cosolvent (see, for example, U.S. Pat. No. 3,898,090). Such in-solvent binders have been shown to suffer the disadvantage of short shelf life ("several days") due to additional silanol condensation during storage. A further disadvantage is that these binders require the step of solvent removal from the core or mold by a drying process ("to remove a major portion of the alcohol-water cosolvent") before metalcasting. Otherwise, voids and poor mold integrity result during the metalcasting process. The use of hardenable, liquid organometallic, ceramic precursors as solventless binders for the fabrication of sand molds, shells, and cores has not been disclosed.